Trimethylene structure

An extreme example of the facilitation of stereomutations through radical stabilization of trimethylene structures is provided by the homopentafulvenes shown in equation 5. Interconversion between the two isomers occurs rapidly at 24 °C (half-time about 45 min), a consequence of the unusually large radical stabilization energy of cyclopentadienyl206. 

As mentioned earlier, the cyclopropane radical cation, prepared by y-radiolysis in rigid matrices, had an ESR spectrum compatible with the trimethylene structure Ai) [78, 79]. Irradiation of several methyl-substituted derivatives at 77 K gave rise... 

The phase behavior of several polybibenzoates with oxyalkylene spacers has been reported [11,14,15,20-27]. These spacers include the dimer of trimethylene glycol and different ethylene oxide oligomers. The most noticeable characteristic of these polybibenzoates with ether groups in the spacer is the considerable decrease of the rate of the mesophase-crystal transformation. Thus, Fig. 8 shows the DSC curves corresponding to a sample of poly[oxybis(trimethylene)p,p -bibenzoate], PDTMB, with a structure similar to that of P7MB but with the... 

The result is explained by considering the stacking structure between the quinoline moiety and the benzene ring linked to the carboxylic acid, which gives the cavity size adequate for Li+. (Fig. 3) Several selective host molecules for Li+ such as [13]crown-4 18), [14]crown-4 19), [16]crown-4 20>, or noncyclic polyether amide derivatives 21) also possess trimethylene moiety, and this is an interesting finding from the point of view of molecular design of new host molecules for Li+. 

Pego AP, Siebum SB, Luyn MJAV, et al. Preparation of degradable porous structures based on 1,3-trimethylene carbonate and D,L-lactide(co)polymers for heart tissue engineering. Tissue Eng, 2003, 9, 981 994. 

These new derivatives were isolated in good yields (60-94%) as high boiling liquids and were fully characterized by NMR spectroscopy (1H, 13C, and 11B) and elemental analysis. The proton NMR of the starting material 1 shows a well-resolved multiplet and quintet for the trimethylene bridge. Upon monosubstitution, however, three complex multiplets are observed, indicative of the unsymmetrical structures of these derivatives. Also, the nonequivalence of the N-C carbon atoms is clearly apparent in the 13C NMR spectra of 2-4. 

On orbital symmetry grounds, the addition of ethylene to ethylene with ring closure (cycloaddition) should be thermally forbidden. If one compares this reaction with the reaction of trimethylene with approaching ethylene and butadiene (Fig.4), it is readily seen that, the A level being below the S level in trimethylene, the behaviour with respect to cycloaddition to olefins is reversed, that is, trimethylene is essentially an anti-ethylene structure. This principle can be generalized for instance (16) ... 

This isomerization is a symmetry-forbidden one, since a level crossing such as the ones found in trimethylene and tetramethylene is found (22) (Fig. 13). It should be noted that, in all these cases, the two-minima structure due to level crossing might, in principle, be swept out by configuration interaction. If the barrier survives, however, the two isomers [4] and [5] can be called bond-stretch isomers. Another example is the isomerization [6] [7] (23). 

Meanwhile Ethicon (and others) developed alternative absorbable surgical sutures, based, for example, on copolymers of polyglycolide with poly-L-lactide or poly(trimethylene carbonate), and on polydioxanone, and on poly(e-oxycaproate), and also on copolymers of these with polyglycolide or with each other. These different structures made it possible to provide fibres with different rates of absorption, with different degrees of stiffness or flexibility, and for use in monofilaments, braided multifilaments, and other yam structures, as required for different surgical operations. 

Figure 11.1 The chemical structure of poly(trimethylene terephthalate)...

Figure 11.14 Effect of applied strain on the 002 d-spacing of a PTT fiber drawn at 3.3 x measured by WAXD [76], Reprinted from Polymer, 42, Wu, J., Schultz, J. M., Samon, K. M., Pangelinan, A. B. and Chuah, H. H., In situ study of structure development in poly(trimethylene terephthalate) fibers during stretching by simultaneous synchrotron small- and wide-angle X-ray scattering, 7141-7151, Copyright (2001), with permission from Elsevier Science...

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene ( Ai) type, r " ... 

DuPont and Shell have developed a new polyester, poly(trimethylene terephthalate) (PTT) (structure 19.38) that is structurally similar to PET, except that 1,3-propanediol (PDO) is used in place of ethylene glycol. The extra carbon in Sorona allows the fiber to be more easily colored giving a textile material that is softer with greater stretch. Further, it offers good wear and stain resistance for carpet use. The ready availability of the monomer PDO is a major consideration with efforts underway to create PDO from the fermentation of sugar through the use of biocatalysts for this conversion. Sorona and Lycra blends have already been successfully marketed. Sorona is also targeted for use as a resin and film. 

The visual and conceptual impact of seeing the timed sequence of structures, a full representation of atomic-scale events as a complex chemical reaction took place, was powerful. This achievement, the product of state-of-the-art calculations applied to an ambitious objective as well as excellent presentation graphics, was not diminished through a repressed awareness that it aU depended on theory. Nothing experimentally based provided an anchor for the visually compelhng rendition of the reacting system as a cyclopropane cleaved a C C bond, formed a trimethylene diradical intermediate, and executed a net one-center epimerization before reverting to the cyclopropane structure. 

The pump-probe-detect arrangements for the femtosecond experiments was similar to those described above. When cyclobutanone was pumped with two photons of a X = 307-nm femtosecond pulse, two consecutive C—CO bond cleavages led to the formation of the trimethylene diradical, detected as an easily ionized transient at 42 amu, with buildup and decay times of 120 20 fs. The decay presumably involves isomerizations to cyclopropane and to propylene— structures not ionized by the probe pulse and thus undetected during the experiment. 

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